Forum: Catapult Message Board

on 09/23/2009 08:00 pm, by Grozny 2 stars

A trebuchet is a type of catapult that converts the potential energy of a counterweight into the kinetic energy of a projectile. The simplest version operates like a see-saw, with the counterweight suspended from a hanger attached to the short arm and the projectile held in a sling attached to the throwing arm. When the short arm is raised and then released, the throwing arm rotates faster because it is longer and the sling rotates even faster as it whips around the end of the throwing arm.

At their inception during the Middle Ages, trebuchets were used to hurl boulders at castle walls in an attempt, usually successful, to batter them down. Modern trebuchet designers, lacking castles to besiege and fair maidens to rescue, must content themselves with hurling cooking pumpkins for distance. Competition, however, is fierce, and the winner of the contest can expect any fair maidens present at the pumpkin festival to hurl themselves at him. Thus, it behooves us to put as much study into the ballistics of pumpkins as old-time mathematicians put into the study of cannonballs.

From wind tunnel tests we know that the coefficient of drag, CD, of a pumpkin-sized cannonball is 0.2. This is not bad, as smaller spheres, such as musket shot, have a CD of 0.45 due to the fact that their Reynolds number is below the critical point where the air flow in the boundary layer changes from a smooth motion to a turbulent one. Somewhat counter-intuitively, a turbulent boundary layer carries more energy and is thus able to remain in contact with the surface a bit longer. This narrows the wake and increases the pressure behind the sphere. The reduction in form resistance more than compensates for the increase in viscous resistance, so there is a net reduction in drag.

Visit The Ballistic Coefficient of Pumpkins (external link)

to read the rest of the paper.

on 09/23/2009 09:49 pm, by Thomas 5 stars

Well, Victor the Terrible, you spent a fair amount of verbiage "immortalizing" the BAT video. I have to challenge your conclusion, though.

You state:

"It is possible that people are not analyzing their own videos. Or, possibly, the fact that their projectiles are released well over their heads gives them the illusion that their projectile’s altitude is due to a steep angle rather than to initial height. But, whatever the cause of the problem, designers need to put more work into tuning their trebs to launch at 45° before committing their inventions to YouTube."

The introductory paragraph to the video reads:

"An experiment in improving trebuchet performance by increasing the dropping distance of the counterweight. This is a rough prototype made from bits and pieces of wood that was on hand and is in a state of development.. A number of issues still need to be dealt with, but it works pretty well."

The stuff actually on ballistic properties of pumpkins is OK, if pretty basic. However, the suggestion that a pumpkin be filled with water for added density would be against the rules as set down by WCPC. Most other events follow the WCPC rules.

"8. Pumpkins are not to be altered in any way, excluding P.C.A. marker paints. All pumpkins must be in their natural state.
Pumpkins fired from machine during competition will be measured from the front of or the farthest point of machine. Your pit #
may be written on your pumpkin to help in the identification at the landing zone."

on 09/23/2009 10:48 pm, by Walker 4 stars

Grozny,

Please stop.
You know not whereof you speak and it's embarrassing.

Walker

on 09/23/2009 11:33 pm, by Grozny 2 stars

> Grozny,
>
> Please stop.
> You know not whereof you speak and it's embarrassing.
>
> Walker

>
> I have seen a pumpkin fired at a depressed angle (below horizon) which climed to a 15*< ascent and continued for about 2,000 feet.
>
> Walker

You say that you saw this with your very own eyes and I believe you.

But I have to ask: Are you sure that you weren't observing the flight of a UFO?

Perhaps a little orange UFU happened to fly by at the very moment that your sling broke and left your pumpkin sitting in the grass six inches in front of your foot?

Because what you describe is really not how projectiles behave here on Earth.

on 09/24/2009 08:39 pm, by madmattd 3 stars

> > Grozny,
> >
> > Please stop.
> > You know not whereof you speak and it's embarrassing.
> >
> > Walker
>
> >
> > I have seen a pumpkin fired at a depressed angle (below horizon) which climed to a 15*< ascent and continued for about 2,000 feet.
> >
> > Walker
>
> You say that you saw this with your very own eyes and I believe you.
>
> But I have to ask: Are you sure that you weren't observing the flight of a UFO?
>
> Perhaps a little orange UFU happened to fly by at the very moment that your sling broke and left your pumpkin sitting in the grass six inches in front of your foot?
>
> Because what you describe is really not how projectiles behave here on Earth.

Um, if he is talking about what I think he is, I was there with Walker. 2007, Bridgeville, DE. Chucky did this. Pretty much flat trajectory that climbed forever. I couldn't believe what I saw, but there you go. MASSIVE Magnus Effect. It is possible, and I have the engineering background to understand it, but it is all very emperical, tough to explain and write down into something applicable for a given geometry and other conditions.

Matt

Matt

on 09/24/2009 03:40 am, by warande 3 stars

flying pumpkins are weard things, i'll give some examples:

a pumpkin fired started to turn sideway's, ending up making an 90° turn ito the wind...

a pumkin fired allmost horizontal started to climb and then dropped like a wounded bird...

the turning sideway's happened often to us, some went left, some went right, with same wind conditions, it all depends on projectile shape, center of gravity and the ammount of spin the sling gave...

there is no way to predict the way it will behave after release, that's the reason why i like hurling a lot, you never know what's gonne happen

on 09/24/2009 08:11 am, by Walker 4 stars

If a Base Ball, thrown at 80 MPH can perform a drop or lift or curve by reason of imparted spin (and the position of the stitches),
Then is it so hard to accept that a pumpkin may also have sufficent spin imparted to it at 120+ MPH whith which to cause positive lift?

UFO photo attached.

Really, Groz', I take it you were trying to disfuze the tension rather than trying to be a jerk with the UFO comment.

If you want UFOs, please visit my website: Evil Genius Labs (external link)

Your computer will be scanned and if no government spyware is found, we will open upon your hello.
However, if your computer is found to contain any form of government spyware, you will be ignored.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Edit: add on 25SEP09.

Grozny (external link)

asked: "It is possible that people are not analyzing their own videos (?)."

Yes, it is possible. That is why a number of us have taken to analyzing each other's works. My video library holds 40 test videos from JohnN and another 37 from Thomas. And others, too.
I edit and reproduce these into slo-mo, super slo-mo, super-duper schlow-mo, etc.
I consider that a fair question.

on 09/24/2009 08:44 am, by trebdude 3 stars

Chuck's last throw at WCPC climbed for about 2-3 seconds after launch, then set a new record. Pumpkins are imperfect and behave weirdly.
Tom

on 09/24/2009 11:42 am, by Thomas 5 stars

Seems like it's Bash Grozny Time. Why? Well, we have someone from Academia telling people who have real-world knowledge of a subject "how it is".

The article in question is basically fluff. Other than data taken from one pumpkin, statistically an invalid sampling, and the previously discussed video insertion, not particularly germane, it's unattributed cribbed data (unless Grozny did the wind tunnel tests himself, in which case I apologize).

Missing data include what variety of pumpkin was tested, of the many available. Some are more suitable for hurling than others, a fact well known to hurlers, apparently not to others.

Among the missing was any evidence that the ballistics of spinning objects was looked into. There is a large amount of research on that subject, much of it by the manufacturers of golf balls. Spin is an important factor in the flight of many objects. "Bend it like Beckham", hooking or slicing a golf ball, a popup in baseball.

Proceeding from that, the effect of the shape of various pumpkins needed to be tested. Does a ribbed one fly better than a smooth one? Dimpled golf balls do better than smooth ones, as is well known. Some pumpkins are flatter than others. Will a flat one thrown edge-on do as well as a round one? With a round pumpkin, is the orientation of the stem important?

When the paper sticks to its subject it's correct, if overly-limited in its treatment of that subject. Unfortunately it strays into other areas and reaches conclusions that are questionable.

on 09/29/2009 09:31 pm, by Grozny 2 stars

> Seems like it's Bash Grozny Time. Why?

Why? Probably because I refuted your theory that 45° is only optimal in a vacuum. Duh!

I added the following paragraphs to my paper just for you:

In the first draft of this paper, I wrote that everybody knows that the optimal launch angle is 45°. I assumed that deviations from this ideal were inadvertent. However, after joining the Catmess (Catapult Message Board), I found that the majority of hurlers, or at least the most vocal ones, earnestly believe that 45° is only optimal in a vacuum. For example, one guy, who has constructed a trebuchet that launches at 24°, writes:

"Trig is something I never used and forgot around 1962. When working with trebs you need to divorce yourself from pure mathematics. The real world is very different.

"Galileo proposed the 45° launch angle, and he was a bright guy, but he didn't know everything. Don't be too much in love with pure mathematics. The 45° thing may be valid for a parabolic flight path, but missiles don't follow parabolas except at low speeds and/or with high ballistic coefficients.

"I've seen a claim that 31° was the optimum launch angle, and the claimant had math to back it up. You know what they say about figures don't lie.''

While these guys are a little fuzzy on their rationale for a smaller angle, it is possible that they have heard that one needs to fire low when faced with a stiff headwind and they thought that the “wind” created by the pumpkin’s motion through still air also necessitates a low shot.

The reason that one fires low into a headwind is that wind is horizontal. While the pumpkin is ascending, the wind pushes it back, steepening its angle of ascent and thus necessitating starting out with a smaller initial angle. But the “wind” created by movement is not horizontal; it pushes in exactly the opposite direction of motion, whichever way that might be. Thus, it does not affect the pumpkin’s angle; it just slows the pumpkin down.

on 09/29/2009 09:35 pm, by Grozny 2 stars

Thanks to you guy's highly accurate and unimpeachable eyewitness accounts, I have able to add the following paragraphs to my paper:

I see no evidence that the opening of a nylon pouch imparts spin to pumpkins. Balls are given spin when they roll; for example, a baseball rolls off the pitcher’s fingertips or a golf ball rolls up the angled face of a club. A golf ball is violently struck by a steeply angled clubface made of unyielding steel and with very aggressive grooves cut into it to gain traction and assure that the ball rolls rather than slides up the face. It accelerates from a standstill up to speeds over 60 m/s in less than 10 ms, which is a very different thing than a pumpkin gently nestled in a smooth nylon pouch and taking about 50 to 60 times longer to accelerate up to speed, at which point the pouch just opens, without any twisting motions. Anyway, pumpkins lack the raised stitches or dimples that cause baseballs or golf balls to curve.

There is a group of guys at the Catmess who seem to think that the phrase “Magnus effect” is a sort of magical incantation that, when spoken, will dispel any doubts about the most outlandish braggadocio. These guys are probably amateur golfers who learned this big and very scientific-sounding word from their golf pro. Observing the rather unpredictable trajectories of their own golf balls at the driving range, they concluded that ballistics has a sort of anything-goes quality that is ideally suited to internet bragging.

For example, one fellow writes, “I have seen a pumpkin fired at a depressed angle (below the horizon) which climbed to a 15° ascent and continued for about 2,000 feet.” When asked for an explanation of this gravity-defying feat, he replies, “Is it so hard to accept that a pumpkin may have sufficient spin imparted to it at 120+ mph with which to cause positive lift?”

Obviously, there is not much that I can do to help this gentleman. But, for those of you launching pumpkins at angles between zero and ninety degrees and with sling pouches that somehow manage to open without spinning the poor pumpkin up to hundreds of rpm, my software will help you create a ballistics table for your trebuchet.

Also, while I hesitate to impugn the craftsmanship that goes into these homemade devices, I must raise the possibility that they wobble. Perhaps sometimes their frames flex to the left and sometimes to the right. The resulting trajectories, being reminiscent of the hook or slice of a golf ball, conjured up talk of the Magnus effect, rather than a tightening of wood screws, which might have actually helped.

A wobbly treb is probably the explanation for this guy’s observations:

"Turning sideways happened often to us; some went left, some went right, with same wind conditions. It all depends on projectile shape, center of gravity and the amount of spin the sling gives. There is no way to predict the way it will behave after release. That's the reason why I like hurling a lot. You never know what is gonna happen."

Frankly, I do not think I would want to stand too close to this guy’s treb during a pumpkin launch. You never know what is gonna happen!

on 09/29/2009 10:43 pm, by admin 3 stars

Hey Grozny,

are you sure you've never been here before? You're starting to sound an awful lot like a guy we used to know named David Ratliff.

Here, try this on for size: "In theory, there is no difference between theory and practice. In practice, however, there is." That's because the world is a much bigger place than our little brains are, and we just can't fit all the variables, permutations, eventualities and complex interactions in them. Knowledge without wisdom can be dangerous.

You need to put your ego aside and get an education. Go talk to any ~~let me repeat that last word... ANY~~ college freshman level engineering physics professor. Ask him about ballistic motion and optimal trajectories in an atmosphere, and you'll find out that it greatly depends on the spin, drag coefficient and velocity of the projectile. Go to any - once again, ANY - hurling competition and watch the pumpkins spin like crazy out of the pouch. Listen to the whistle of the bowling balls, and tell me that thing ain't spinning. I dare you.

The velocity of the projectile changes as it flies. It's DE-celerating due to the friction of the air. That means that the interaction between the vertical and horizontal components of the trajectory interact differently at the beginning than at the end, and you need a lower angle to get the most distance. That 45 degree nonsense only works in a vacuum because the horizontal component must remain constant (no drag) for it to work out that way.

Oh, also, if you post another one of your self-promoting inflammatory spam-like posts to my web site, I'll ban you. Please stay, we like intelligent and lively discourse, but please put the ego aside. We know you mother believes in you, but out here, you've got a lot to learn. You've been warned.

-- Ron

on 09/30/2009 08:56 pm, by Grozny 2 stars

> Hey Grozny,
>
> are you sure you've never been here before? You're starting to sound an awful lot like a guy we used to know named David Ratliff.
>

I am not David Ratliff and I don't know him.

on 09/30/2009 08:58 am, by SpringTreb 3 stars

The further the attachment point of the sling is from the release pin/ring, the more spin is imparted on the projectile. As the sling and projectile rotate around the TA tip, the pouch rolls the projectile like a baseball pitcher fingers do on a baseball.

Craig

> I see no evidence that the opening of a nylon pouch imparts spin to pumpkins.

on 09/29/2009 10:08 pm, by trebdude 3 stars

You are more than welcome to come to Delaware and meet some of the fine people that have no idea what they are talking about. Of course it would be easier to sit behind a computer and criticize those that have actually constructed something besides a "paper" and some sarcastic comments.

on 09/29/2009 11:24 pm, by Walker 4 stars

Groz',
Tact isn't your forte, is it.

If you have some credentials, please share.
It may make it easier to accept some guy who apparently walks in off the street and starts spouting off about how we are all wrong even though his head is so far up his inseam that his eyes have turned brown.

My creds? None you would recognize until one hit you.

Your "Paper"?
I found that the selected quotations of my fellows here tended to elevate the quality of the work.
I am quoted accurately if not attributed. (Nor do I consent to being quoted outside of this forum.)

This “gentleman” (hack, hack) did not ask for your misguided help. Furthermore I find it insulting to be labeled as such.
Fact is, Groz’, I am not a gentleman at all. I’m one of those guys who volunteered to fight so that your right to express yourself would be protected. That being said, I really hope you will find another forum in which to express yourself.

Walker

on 09/29/2009 11:25 pm, by madmattd 3 stars

How about a 5th year Mechanical Engineer who has some Fluid Dymanics under his belt? The Magnus Effect is REAL, I have personally observed its effects with trebuchets, caused it to occur even more by adjusting the fixed position of the sling, and can even give you some rough idea of its effects on a pumpkin (way too long to go into right now, and it's late). And the 45 degree trajectory, as ANY Physics Teacher who knows what they are talking about will tell you, applies only in a vaccuum. In fact, most state that when they introduce the "optimum" trajectory as 45 degrees, mine sure did. Again, it has to do with some Fluid Mechanics/Dynamics. Please, I do not want to sound like I am attacking you, but I cannot stand people standing behind a desk/computer telling people who have real-world experience what is real and what isn't. Theory is great, but you need practical hands-on experience to back it up and to make yourself credible. I know far too many people from my time in college that do not understand that the real world does not work according to their equations.

Matt

on 09/30/2009 10:46 pm, by Walker 4 stars

When theory and practice colide, which is the first to go?
Groz' does not understand Zen Trebuchet.
Therefore all he has is what he remembers from his classes.

GROZNY (external link)

Walker

on 10/27/2009 08:25 pm, by BobP 3 stars

This got posted last week on another board I frequent. Here's a link (external link)

.

Conclusions are generally similar to here. You can probably guess which member I am over there...

on 10/27/2009 10:48 pm, by thomas 5 stars

> This got posted last week on another board I frequent. Here's a link (external link)

.
>
> Conclusions are generally similar to here. You can probably guess which member I am over there...

Bob, thanks for bringing that to our attention. Maybe I'm a little too close to the situation, but Grozny/Shaka chose to misrepresent data regarding the video cited (mine) and refused to change that misrepresentation in his paper when I called him on it. The dude lacks intellectual honesty.

He got well and truly spanked on the other forum, and while it may be uncharitable for me to be glad, I am. I see that he hadn't made a follow-up to his original post. Didn't get his desired response so he's all in a snit, I guess.

on 10/28/2009 09:06 am, by trebdude 3 stars

That one guy basically went line by line disproving his paper. Remind me never to submit anything to rocket scientists! Ouch! eek

on 10/28/2009 11:07 am, by madmattd 3 stars

> That one guy basically went line by line disproving his paper. Remind me never to submit anything to rocket scientists! Ouch! eek

I was glad to see he did that. No spin on pumpkins, no way. Should've seen our last test shot Sunday, you wanna talk about spin...

Matt

on 10/28/2009 01:30 pm, by BobP 3 stars

> That one guy basically went line by line disproving his paper. Remind me never to submit anything to rocket scientists! Ouch! eek

The BAUT forum is pretty frequently visited by folks who believe the moon landings were faked and other even wackier stuff. The members are pretty used to using facts to destroy speculation. Ron Toms has even posted a message now, but nothing more from Shaka/Grozny in either location.

on 10/31/2009 01:29 pm, by Walker 4 stars

> That one guy basically went line by line disproving his paper. Remind me never to submit anything to rocket scientists! Ouch! eek

A scientific paper deserves scientific reasoning.
Mr. Grozny is unable to contribute scientifically and must resort to character assinations and misrepresentations (AKA: "Lies").

I just returned to the site (external link)

and found that Ron had been quoted:
"You need a lower angle to get the most distance. That 45° nonsense only works in a vacuum because the horizontal component must remain constant (no drag) for it to work out that way. Go talk to any — let me repeat that last word... ANY — college freshman-level engineering physics professor. I dare you."

– Ron Toms
(Administrator, Catapult Message Board)

And supposedly rebutted by a quote from a "professor":

"It will be noticed that the maximum value of v2sin(2φ)/c for a given velocity, v, and ballistic coefficient, c, occurs when φ = 45°. This means that maximum range will be found with this angle of projection, which is not strictly true, although, in practice, variations of φ around this value produce very little effect on the range."

– Professor C. B. Daish
(The Physics of Ball Games, p. 128)

I searched for anything on "Professor" Daish but could not find where he taught or of what subject he was said to be a professor.
In spite of lacking internet bona fides, he has written a few books with over simplified explinations of the physics involved with ball related sports.
May I presume the professor does not teach AQP?

While I do not accept a quote from a professor as being the same as the professor actually responding to Ron's challange, I don't think this professor is still able to respond for himself.

Further exploation shows this quote, three quarters of the way down the page:

"The optimal angle is actually 42.5°".

No Duh, Sherlock?
Thanks for waisting all our time, Grozny.
Now just go away.

Walker

on 10/31/2009 06:50 pm, by BobP 3 stars

> Thanks for waisting all our time, Grozny.
> Now just go away.
>
> Walker

He already has. Hasn't been back to the other forum either. But there've been some good discussions there, including visits from Ron & Ripcord.

on 11/01/2009 05:26 am, by Walker 4 stars

>
> > Thanks for waisting all our time, Grozny.
> > Now just go away.
> >
> > Walker
>
> He already has. Hasn't been back to the other forum either. But there've been some good discussions there, including visits from Ron & Ripcord.

Thank you Bob.

Once again, here's a link to the BAUT forum. (external link)

Walker

on 03/17/2010 07:50 pm, by SpringTreb 3 stars

Punkins do spin.

http://www.youtube.com/watch?v=Blftpqd4Gd4 (external link)

Craig

on 03/17/2010 08:12 pm, by Thomas 5 stars

> Punkins do spin.
>
> http://www.youtube.com/watch?v=Blftpqd4Gd4 (external link)

>
> Craig

No question to any of us here that they spin. That looked like a futbol, though. I wonder if Grozny would deny that Beckham "bending it" is anything but observer error. No raised seams or dimples, ya know.

Funny how someone with zero hands-on experience can set himself up as an expert.

Thomas

on 03/17/2010 10:26 pm, by SpringTreb 3 stars

I thought it was a pumpkin until I finally saw the end of the video. I couldn't get it to play more than halfway thru earlier.

Craig

> > Punkins do spin.
> >
> > http://www.youtube.com/watch?v=Blftpqd4Gd4 (external link)

> >
> > Craig
>
> No question to any of us here that they spin. That looked like a futbol, though. I wonder if Grozny would deny that Beckham "bending it" is anything but observer error. No raised seams or dimples, ya know.
>
> Funny how someone with zero hands-on experience can set himself up as an expert.
>
> Thomas

on 03/18/2010 09:23 am, by trebdude 3 stars

More than likely, it's a fitness ball. We use one for testing, weighs exactly 8 lbs, is about the right size and takes a beating without getting destroyed in the process. A good $15-$20 investment.

on 03/18/2010 02:31 pm, by madmattd 3 stars

> More than likely, it's a fitness ball. We use one for testing, weighs exactly 8 lbs, is about the right size and takes a beating without getting destroyed in the process. A good $15-$20 investment.

Yea. We pegged our 4-pounder into a tree at about 550-600 feet, on a line drive. Bounced straight back out a good 100+ feet! And you wouldn't even know it by looking at it. Those things are great.

on 03/19/2010 07:40 am, by Walker 4 stars

I was hesitant to see ugly slime may have returned to this thread but I only found my hurling buds, nobody named after a Chechen city that the Russans used like a cheap ~um~ (family safe site?) ~er~ Ah!:
like a cheap plate of HUNGARIAN GOULASH!

Maybe he was abducted by alien pumpkins.
Maybe someone would think I had anything to do with his sudden disappearance. Who? Me?
I don't make goulash, I do Chili. (Non-Texas style.)

Footballs don't spin, either.

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The Ballistic Coefficient of Pumpkins

"The optimal angle is actually 42.5°".

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